A conventional power supply apparatus that supplies DC power to a load is shown in Japanese Patent Application Laid-Open Publication No. H9-47024. A circuit diagram of the conventional power supply apparatus is shown in FIG. 6. The power supply apparatus 400 shown in FIG. 6 is configured as a step-down AC-DC convertor having a full-wave rectifier circuit 410, a low-pass filter 480, a power supply circuit 430 for supplying DC power to a load 420, and a control circuit 440 for controlling the power supply circuit 4300. The full-wave rectifier circuit 4100 converts an AC voltage Vac of an AC power source 10 to a DC voltage (pulsating DC voltage) Vdc obtained by full-rectifying the AC voltage Vac. An N-type MOSFET is used as a switching element T431 of the power supply circuit 430. The power supply circuit 430 has a power factor correction (PFC) function for suppressing an amplitude of a harmonic current less than a limit value (referred to as “PFC Standard”). The control circuit 440 turns on the switching element T431 in a fixed control period and turns off the switching element T431 when a drain current of the switching element T431 (voltage drop across a current detecting resistor R431) becomes larger than a threshold value.
In the power supply apparatus 400 shown in FIG. 6, when the switching element T431 is turned on, a current I1 flows through a path formed by the full-wave rectifier circuit 410, the low-pass filter 480, an inductor L431, a parallel circuit consisting of the load 420 and a capacitor C431, the switching element T431, the current detecting resistor R431, and the earth (ground). At this time, electromagnetic energy is accumulated in the inductor L431 (inductance L). When the switching element T431 is turned off, a flywheel current I3 caused by the electromagnetic energy accumulated in the inductor L431 flows through a path formed by the inductor L431, the parallel circuit consisting of the load 420 and the capacitor C431, a diode D431 (referred to as “flywheel diode”).
In the power supply apparatus 400 shown in FIG. 6, a magnitude (amplitude) of the current I1 is variable in accordance with a magnitude (amplitude) of the DC voltage Vdc. Thus, a power factor cos θ of AC input power (θ: phase difference between AC voltage Vac and AC input current Iac) approaches “1”.
The current I1 does not flow while the switching element T431 is off. That is, the current I1 intermittently flows. Therefore, harmonic currents are included in the current I1. A low-pass filter 480 is provided to prevent the harmonic currents from propagating to the AC power source side.